A review of recent advances in molecular assembly of liquid crystalline chiral photonic crystals is presented, particularly on fabrication of extraordinarily thick [>2 mm] cholesteric liquid crystals (CLC), and large-areal size monocrystalline Blue-phase liquid crystals (BPLC). CLC’s as 1-D chiral photonic crystals possess large ultrafast optical nonlinearity suitable for self-compression, polarization switching and modulation of complex vector beams. These super thick and highly nonlinear CLC’s are estimated to be capable of all optical switching [π- phase shift] at sub-picosecond (~0.1 x 10-12 s) speed with low threshold energy fluence of ~ 0.25 μJ/cm2. Large areal size monocrystalline BPLC’s will preserve the phase front uniformity in free-space image processing and yield better efficiency, resolution, and image/signal qualities than their typical polycrystalline counterparts.
KEYWORDS: Transverse mode instability, Multimode fibers, Wavefronts, Simulations, Laser systems engineering, High power fiber lasers, Gaussian beams, Gain switching, Fiber amplifiers, Complex systems
By coherently seeding a multimode fiber amplifier with a tailored spatial wavefront, it is possible to generate the target output profile in the presence of thermo-optical nonlinearity and gain saturation. We prove numerically that such wavefront exists with a modified phase-conjugation approach. An optical beam with target profile and power is launched from the distal end to proximal end of a multimode waveguide with absorption, and the exiting wavefront is phase-conjugated and sent back to the waveguide with loss switched to gain of equal magnitude. The target output beam is obtained with enhanced threshold for transverse mode instability due to multimode excitation.
We experimentally demonstrate a single-frequency Yb-doped multimode fiber amplifier (~76 modes) with a focused Gaussian output beam and enhanced stimulated-Brillouin-scattering (SBS) threshold. With few-mode excitation, the SBS threshold output power (57 W) is already an order of magnitude higher than that for a single-mode fiber amplifier of the same length. The output beam, however, is slightly speckled. To achieve high beam quality, we optimize the input wavefront to focus output light to a single diffraction-limited spot and simultaneously achieve much higher SBS threshold (up to 105 W), because forming a tight output focus requires coherent amplification of many modes.
We report a room-temperature dual-frequency field assembly technique that is capable of fabricating large-areal size (~cm2 or larger), well-aligned cholesteric liquid crystals to thicknesses up to 2.2 mm, corresponding to period number N (thickness/index grating period) of nearly10,000 in the visible spectral regime. The method employs successive application of low- and high- frequency electric field on a thick cell of CLC starting mixture containing a nematic constituent of negative anisotropy. The low-frequency field creates conductive hydrodynamical instabilities that mash the mixture to a state with completely randomized orientation of the cholesteric helices; the next application of a high-frequency field at a field strength below the dielectric hydrodynamic instability reorient all the helices into uniform standing helices. Such extraordinarily thick chiral photonic crystals exhibit many never-before-realized chiral photonic properties such as giant rotation of optical polarization with high transmission (low scattering loss), polarization switching and ultrafast pulse modulation capabilities for visible to mid-infrared lasers, in addition to dynamic tunability by electrical, thermal, or optical means.
Multimode fibers provide a promising platform to efficiently suppress Stimulated Brillouin Scattering (SBS) by controlling input excitation. We demonstrate SBS suppression can be formulated as a problem of optimization of the input power distribution among the fiber modes. We provide a method to obtain the optimal power distribution based on linear programming. The SBS growth rate depends linearly on the input power distribution, allowing us to map SBS suppression into a constrained linear optimization, solvable numerically. We show that for a highly multimode step index fiber, optimal input excitation gives 9.5 times higher SBS threshold compared to fundamental mode-only excitation.
Transverse Mode Instability (TMI) is one of several nonlinear effects that limit power scaling in high power fiber lasers and amplifiers. We demonstrate that TMI can be effectively suppressed by spreading power in multiple modes of the fiber. We show that the TMI threshold scales linearly with the number of modes, upon equal excitation of modes, caused by smearing of thermally induced refractive index grating. The multimode excitation can be focused to a diffraction limited spot, giving high quality beam with increased TMI threshold. We finally show linear scaling of TMI threshold is maintained upon inclusion of gain saturation.
An electrically tunable achromatic polarization rotator has been developed based on the hybrid splay-twist (HST) and hybrid-aligned super twist (HAST) liquid crystal. The continuous angular rotation, and achromatic operation across the entire visible spectrum. The tuning range of the polarization rotator is up to 90° or to 180° and the degree of linear polarization (DOLP) remains. Based on the HST-LC, the multi-functional smart glass is realized with light field, dimming and scattering control. This work provides possibilities in the design of optical systems and spatially polarization multiplexing elements. The designed smart glass provides novelties in smart (green) architecture.
By virtue of their constituents’ soft matter attributes and easy susceptibilities to applied fields, liquid crystalline chiral photonic crystals exhibit highly tunable linear and nonlinear optical properties that allow applications throughout a very wide spectrum. In particular, nearly-mm thick 1-D chiral photonic crystals fabricated with cholesteric liquid crystal are shown to be capable of polarization rotation and switching of complex laser vector fields with response times that can be as fast as sub-picoseconds, and operating wavelength ranging from the visible to near- and mid-IR range. [Ref.: Nature Comm. 8, Article number: 727 (2017); PNAS 2021, 118 (16)].
Stimulated Brillouin Scattering (SBS) provides a major limitation on power scaling in high power fiber lasers and amplifiers. Using wavefront shaping in highly multimode fibers provides a promising avenue to suppress SBS while maintaining good beam quality. We present here a generalized theory for SBS in multimode fiber amplifiers. We find the Stokes susceptibility in terms of eigenmode expansions of the vector optical and acoustic wave equations. An analytical form of the relevant gain matrix is obtained in terms of modal overlap integrals. We will discuss wavefront shaping strategies to suppress SBS based on the properties of the gain matrix.
Under an applied electric field, BPLC lattice undergoes with complex reconfiguration dynamics and exhibit meta-stability and new crystal symmetries. Detailed theoretical considerations and experimental results with such Repetitively Applied Field (RAF)Technique for transforming BPLC crystalline lattice structures from cubic to final stable configuration with orthorhombic or tetragonal symmetry will be presented.
An electrically tunable achromatic polarization rotator has been developed based on the hybrid splay-twist (HST) liquid crystal. The proposed polarization rotator is advantageous over the conventional ones owing to the thin thickness (sub-100μm), continuous angular rotation, and achromatic operation across the entire visible spectrum. The tuning range of the polarization rotator is up to 90° via a simple electric field application; meanwhile, the degree of linear polarization (DOLP) remains. The rotation angle can be expanded to 180° by a tandem-cell geometry. The work will offer possibilities in the design of various optical systems and spatially polarization multiplexing elements.
We present the results from recent studies of field-induced reconfiguration of defect network in Blue phase liquid crystals leading to the formation of new stable lattice structures from their natural self-assembled cubic form [Nat. Materials https://www.nature.com/articles/s41563-019-0512-3]. The dynamical evolution of the defect network and reorientation provide new insights into the underlying mechanisms and roles played by various factors, especially the form of applied field for efficient lattice transformation. Recent studies with optical field derived from CW or short-pulsed lasers further demonstrate the possibility of direct reconfiguration/reorientation of the bulk crystals.
We have succeeded in fabricating unusually thick (up to ~ 550 microns), well aligned cholesteric liquid crystals that possess low scattering loss, large operating temperature range and well-defined photonic bandgap in the visible - near infrared regime. These CLC’s possess sufficiently large ultrafast (sub-picosecond) electronic optical nonlinearity needed for direct compression, stretching and recompression of femtoseconds-picoseconds laser pulses without additional optics, as demonstrated by theory and experiments. Despite such world-record setting thickness, these CLC’s are extremely compact in comparison to other state-of-the-art materials/devices used for similar operations. They are therefore highly promising for miniaturization and reduced complexity of photonic platform/systems for ultrafast pulse modulations.
We have investigated the mechanisms responsible for nonlinear optical processes occurring in azobenzene-doped blue phase liquid crystals (BPLC), which exhibit two thermodynamically stable BPs: BPI and BPII. In coherent two wave-mixing experiments, a slow (minutes) and a fast (few milliseconds) side diffractions are observed. The underlying mechanisms were disclosed by monitoring the dynamics of grating formation and relaxation as well as by some supplementary experiments. We found the photothermal indexing and dye/LC intermolecular torque leading to lattice distortion to be the dominant mechanisms for the observed nonlinear response in BPLC. Moreover, the response time of the nonlinear optical process varied with operating phase. The rise time of the thermal indexing process was in good agreement with our findings on the temperature dependence of BP refractive index: τ(ISO) > τ(BPI) > τ(BPII). The relaxation time of the torque-induced lattice distortion was analogue to its electrostriction counterpart: τ'(BPI) > τ'(BPII). In a separate experiment, lattice swelling with selective reflection of <110> direction changed from green to red was also observed. This was attributable to the isomerization-induced change in cholesteric pitch, which directly affects the lattice spacing. The phenomenon was confirmed by measuring the optical rotatory power of the BPLC.
We report on the investigation of random lasing in blue phase liquid crystals. Multiple scattering and interference effects arising from disordered platelet texture as well as index mismatch between polymer and mesogen contribute the optical feedbacks towards laser action. In pure blue phase liquid crystals, the random laser can be switched between the coherent and incoherent types by executing distinct heating/cooling cycles; and, the randomness of lasing wavelengths can be determined by the platelet size, which can be set by controlling the cooling rate. After the blue phase liquid crystals are polymer-stabilized, coherent random lasing may occur in both the blue phase with an extended temperature interval and the isotropic liquid state; also, the selected modes are constant from one pulse to another. Additionally, if the laser dye is sensitive to temperature, the excitation threshold and the emission spectrum could be altered via thermal control.
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